Electrical power generation and its subsequent use is ubiquitous in modern society. In many societies, so called “renewable” power generation technologies make up a large and growing proportion of the requisite power generation capacity. Renewable power generation technologies include, for example, photovoltaic power generation, wind power generation and tidal power generation.
Most renewable power generation systems are made up of discrete power generation blocks, typically connected in series or parallel. For example, in photovoltaic power generation, multiple photovoltaic cells may be connected together to provide a single output.
Photovoltaic cells are in essence light dependent solid-state rectifiers i.e.: photodiodes. When photons with a specific frequency strike a photovoltaic cell, electrons and holes are freed from the lattice and move towards the electrodes due to the internal electric field created by the intrinsic voltage bias. This voltage bias is normally between 0.5 and 0.7 V in conventional photovoltaic cells.
It is well known that the voltage produced in a photovoltaic cell is independent of the surface area of the photovoltaic cell (see for example U.S. Pat. No. 4,529,832). However, the current produced by a photovoltaic cell is proportional to the surface area of the photovoltaic cell. Whilst increasing the size of a photovoltaic cell will significantly increase its power output, there are practical and manufacturing difficulties in making ever larger photovoltaic cells. Therefore, power output is increased in photovoltaic power generation systems by connecting multiple photovoltaic cells together.
Typically, photovoltaic cells are connected together in either a series relationship or a parallel relationship. In photovoltaic systems where the cells are connected in series, the current is limited to that provided by an individual photovoltaic cell and the voltage is the sum of the voltages of each photovoltaic module in the system. This is reversed in photovoltaic systems where the cells are connected in parallel, i.e. the voltage is limited to that provided by an individual photovoltaic cell and the current is the sum of the currents of each photovoltaic module in the system.
At present the majority of photovoltaic power generation systems in use are based on series connected photovoltaic cells, a design originally developed by Chapin et al. (for example, U.S. Pat. No. 2,780,765). In such a system, the vast majority of the photovoltaic cells are needed only to produce a high voltage and do not contribute an increase in electrical current of the system. As the required output power increases and the photovoltaic system size increases, the amount area of the photovoltaic system producing a current in proportion to the incident light rapidly reduces. To illustrate this, the following example illustrates such a photovoltaic system arrangement:
The characteristics of a typical silicon photovoltaic cell are: 8.5 A maximum power current output, 0.5 V maximum power voltage output, for a surface area of 15.6×15.6 cm2 (receiving 100 mW/cm2 at air mass coefficient (AM) 1.5). Therefore, 72 of these photovoltaic cells must be connected in series to produce 36 V at maximum power point open circuit condition, whilst the current is constant at 8.5 A at short circuit condition maximum power point (equal to the value generated by a single photovoltaic cell). Thus, the actual area producing current in proportion to the amount of received light is just 1/72th of the photovoltaic system, whilst 71 out of the 72 photovoltaic cells occupy space only to generate voltage independently from the amount of light present.
The above example illustrates that significant surface area in prior art photovoltaic systems is not fully utilised. In many cases, the surface area available for renewable power generation is limited or there is no space in which to expand current installations. Therefore, there exists a clear need to increase the power output per unit area of renewable power generation systems. Even in cases where space is not a restriction, there is a clear advantage to being able to increase the power output per unit area.